The present invention is generally directed to toner compositions and processes thereof, and more specifically to overcoated toner compositions and processes for directly generating toner compositions without resorting to the conventional pulverization and classification methods. In one embodiment, the present invention relates to toner compositions comprised of a core comprised of a polymer resin and colorants, including color pigments, dyes, or mixtures thereof, and an outer coating layer comprised of a cellulose component, such as methyl cellulose, hydroxypropyl cellulose, hydroxyethylmethyl cellulose, and the like, and wherein the cellulose layer functions primarily as a protective coating for the core components, especially during the preparation thereof. In addition, with the outer cellulose layer, the toners prepared by the processes of the present invention do not require in embodiments additional protective coatings for mechanical integrity, or for protection against the environment. In one embodiment, the present invention provides toner compositions wherein the outer cellulose coating has been chemically treated. The processes of the present invention do not utilize such toxic and undesirable reagents as polyisocyanates, polyacyl halides, and polyamines, which materials are often the shell precursors for many of the prior art encapsulated toner compositions. The processes of the present invention in embodiments thereof are comprised of an initial dispersion step for forming a stabilized organic microdroplet suspension in an aqueous medium containing a cellulose surfactant, such as hydroxyethylmethyl cellulose, methyl cellulose or the like; a physical overcoating step comprising precipitation of cellulose molecules around the microdroplets; and a final core resin formation step by free radical polymerization. The precipitation of cellulose molecules is believed to begin at the initial dispersion-stabilization stage, and continues during the core resin forming free radical polymerization step. In embodiments, the processes of the present invention can also utilize a combination of cellulose polymers and inorganic surfactants, such as potassium oleate, sodium dodecyl sulfate, and the like during the dispersion step. The cellulose-inorganic surfactant system facilitates efficient generation of very small sized microdroplets, particularly those with an average particle diameter of from about 2.5 microns to about 7 microns, together with a narrow particle size distribution of less than 1.35. The processes of the present invention therefore offer in embodiments simplicity, efficiency, the attributes of utilizing only nontoxic materials; and no post-reaction waste treatments are accordingly necessary. In contrast, a number of known encapsulated toners are comprised of condensation polymer shells prepared, for example, by interfacial polycondensation of toxic polyisocyanate, or polyacyl halides with polyamines or polyols, and costly post-reaction waste treatments are generally required for the preparation of these toner compositions.
The primary function of the cellulose coatings for the toner compositions prepared by the processes of the present invention is to provide stability to the particles during the core resin-forming preparation. The cellulose coatings in embodiments of the present invention also provide mechanical integrity to the toner compositions, and ensure effective containment and protection of the core components. In addition, the coatings also inhibit toner particles from coalescing and prevent, or minimize toner agglomeration. Another important function of the coatings relates to the nullification, or passivation of the triboelectric charging effects of colorants present in the toner compositions, such that the triboelectric charging characteristics of the toner compositions are primarily controlled or dominated by the charging effects of the cellulose layer, and surface additives. The processes of the present invention in embodiments accomplishes many of the objectives illustrated herein without utilizing the usual chemical shell-forming polycondensation reactions. Accordingly, the processes of the present invention are useful for the preparation of a wide variety of colored toners possessing similar or substantially similar triboelectric charging characteristics against a selected carrier, irrespective of the nature of the colorants present in the toners. For single component development where triboelectric charging is generally accomplished by a frictional charging blade, similar equilibrium triboelectric charge levels can also be obtained under identical conditions with different colored toners prepared by the processes of the present invention. Effective coverage of core components by the cellulose coating of the present invention also inhibits the diffusion of core components, thereby eliminating or substantially reducing the problem of toner blocking or agglomeration in toners wherein core resins of low glass transition temperatures are utilized. The cellulose coating for the toner compositions obtained by the processes of the present invention are in general relatively thin in nature, its presence therefore does not substantially affect the toner's fusing characteristics.
In color reprography, such as in full color or highlight color applications, colored toners with a wide variety of colors including black are usually employed. For two component development, it is highly desirable that the triboelectric properties of different colored toners be desirably controlled so that they all attain similar equilibrium triboelectric charging levels when utilized against a selected carrier. This is especially useful for custom colored toner packages, since colored toners with a wide variety of custom colors can be obtained by simple blending of the primary colored toners. Another important aspect for two component development is the rate of charging of the fresh toners to the equilibrium charge levels when they are added to the toner depleted development housing. A fast rate of charging of fresh toner can be important in ensuring proper image development, particularly for high speed reprographic systems.
It is known that color pigments or dyes present in the toner have a dominant effect on the toner's triboelectric charging behavior, arising primarily because these colorants are often also present at or close to the surface of the toner, and are, therefore, exposed to their environments. As a consequence, when the toner particles are admixed with carriers, the interactions of the exposed pigments of the toners with the carrier particles drastically affect the charging behavior of the toner. Similar effects are obtained for a number of prior art encapsulated toners where the color pigment particles are not completely encapsulated within the toner shell. Thus, it is often observed that toners with identical components, except colorants, exhibit different charging behavior, even to the extent of having triboelectric charges of opposite polarity. To overcome this difficulty, it is usually necessary to utilize different charge control additives for different colorants, or to use high levels of charge control additives so as to nullify or overcome the different charging effects of different colorants, and exert a dominating influence on the charging characteristics of the toners. The toners and processes of the present invention eliminate or overcome this difficulty through complete or substantially complete encapsulation of core components since the adsorption of cellulose polymers, and their subsequent precipitation occur on the surface of the microdroplets. As a consequence, the need to rely on different or high levels of charge control additives for different colored toners for achieving similar triboelectric charging levels is eliminated or substantially avoided. Other advantages associated with the toner compositions obtained by the processes of the present invention include, for example, rapid triboelectric charging rates, small toner size and narrow size distribution for high resolution images, excellent color mixing properties and image color fidelity, high image projection efficiency enabling their use on transparent substrates, lower fusing temperatures, acceptable powder flow, and nonblocking and nonagglomerating characteristics. The toner compositions of the present invention can be selected for a variety of known imaging processes including electrophotographic and ionographic processes. Preferably, the toner compositions are selected for electrophotographic processes wherein image fixing is accomplished by heat fusion.
Encapsulated toners and processes are known. For example, both U.S. Pat. No. 4,626,489 and British Patent 1,538,787 disclose similar processes for colored encapsulated toners wherein both the core resin and shell materials are prepared by suspension polymerization techniques. U.S. Pat. No. 4,565,764 discloses a colored microcapsule toner comprised of a colored core encapsulated by two resin shells with the inner shell having an affinity for both the core and the outer shell materials; U.S. Pat. No. 4,727,011 discloses a process for preparing encapsulated toners which involves a shell forming interfacial polycondensation and a core binder forming free radical polymerization; and U.S. Pat. No. 4,708,924 discloses the use of a mixture of two polymers, one having a glass transition temperature in the range of -90.degree. C. to 5.degree. C., and the other having a softening temperature in the range of 25.degree. C. to 180.degree. C., as the core binders for a pressure fixable encapsulated toner. Other representative United States patents are: U.S. Pat. No. 4,339,518, which relates to a process of electrostatic printing with fluorinated polymer toner additives where suitable materials for the dielectric toner are thermoplastic silicone resins and fluorine containing resins having low surface energy; U.S. Pat. No. 4,016,099, which discloses methods of forming encapsulated toner particles and wherein there are selected organic polymers including homopolymers and copolymers such as vinylidene fluoride, tetrafluoroethylene, chlorotrifluoroethylene, and the like; U.S. Pat. No. 4,497,885, which discloses a pressure fixable microcapsule toner comprising a pressure fixable component, a magnetic material, and other optional components, and wherein the core material can contain a soft material typical examples of which include polyvinylidenefluoride, polybutadiene, and the like; U.S. Pat. No. 4,520,091 discloses an encapsulated toner with a core which comprises a colorant, a dissolving solvent, a nondissolving liquid and a polymer, and may include additives such as fluorine containing resin; and U.S. Pat. No. 4,590,142 relating to capsule toners wherein additives such as polytetrafluoroethylenes are selected as lubricating components. Furthermore, there are disclosed in the prior art encapsulated toner compositions containing costly pigments and dyes, reference for example the color photocapsule toners of U.S. Pat. Nos. 4,399,209; 4,482,624; 4,483,912 and 4,397,483.
The following U.S. patents located in a patentability search report for encapsulated toners are mentioned: 3,967,962 which discloses a toner composition comprising a finely divided mixture comprising a colorant and a polymeric material which is a block or graft copolymer, including apparently copolymers of polyurethane and a polyether (column 6), reference for example the Abstract of the Disclosure, and also note the disclosure in columns 2 and 3, 6 and 7, particularly lines 13 and 35; however, it does not appear that encapsulated toners are disclosed in this patent; U.S. Pat. No. 4,565,764 which discloses a microcapsule toner with a colored core material coated successively with a first resin wall and a second resin wall, reference, for example, the Abstract of the Disclosure and also note columns 2 to 7, and particularly column 7, beginning at line 31, wherein the first wall may comprise polyvinyl alcohol resins known in the art, including polyurethanes, polyureas, and the like; U.S. Pat. No. 4,626,490 contains a similar teaching as the '764 patent and more specifically discloses an encapsulated toner comprising a binder of a mixture of a long chain organic compound and an ester of a higher alcohol and a higher carboxylic acid encapsulated within a thin shell, reference the Abstract of the Disclosure, for example, and note specifically examples of shell materials in column 8, beginning at line 64, and continuing on to column 9, line 17, which shells can be comprised, for example, of polyurethanes, polyurea, epoxy resin, polyether resins such as polyphenylene oxide or thioether resin, or mixtures thereof; U.S. Pat. Nos. 4,442,194 and 4,465,755, mentioned herein; and U.S. Pat. Nos. of background interest include 4,520,091; 4,590,142; 4,610,945; 4,642,281; 4,740,443 and 4,803,144.
Furthermore, other prior art, primarily of background interest, includes U.S. Pat. Nos. 4,254,201; 4,465,755 and Japanese Patent Publication 58-100857. The Japanese publication discloses a capsule toner with high mechanical strength, which is comprised of a core material including a display recording material, a binder, and an outer shell, which outer shell is preferably comprised of a polyurea resin. In the '201 patent, there are disclosed encapsulated electrostatographic toners wherein the shell material comprises at least one resin selected from polyurethane resins, a polyurea resin, or a polyamide resin. In addition, the '755 patent discloses a pressure fixable toner comprising encapsulated particles containing a curing agent, and wherein the shell is comprised of a polyurethane, a polyurea, or a polythiourethane. Moreover, in the '201 patent there are illustrated pressure sensitive adhesive toners comprised of clustered encapsulated porous particles, which toners are prepared by spray drying an aqueous dispersion of the granules containing an encapsulated material.
Also, in U.S. Pat. No. 4,599,271, the disclosure of which is totally incorporated herein by reference, there are illustrated microcapsules obtained by mixing organic materials in water emulsions at reaction parameters that permit the emulsified organic droplets of each emulsion to collide with one another, reference the disclosure in column 4, lines 5 to 35. Examples of polymeric shells are illustrated, for example, in column 5, beginning at line 40, and include isocyanate compounds such as toluene diisocyanate, and polymethylene polyphenyl isocyanates. Further, in column 6, at line 54, it is indicated that the microcapsules disclosed are not limited to use on carbonless copying systems; rather, the film material could comprise other components including xerographic toners, see column 6, line 54.
Illustrated in U.S. Pat. No. 4,758,506, the disclosure of which is totally incorporated herein by reference, are single component cold pressure fixable toner compositions, wherein the shell selected can be prepared by an interfacial polymerization process. Further in another U.S. Pat. No. 5,043,240 (D/89069), the disclosure of which is totally incorporated herein by reference, there are illustrated encapsulated toners with a core comprised of a polymer binder, pigment or dye, and thereover a polymeric shell, which contains a soft and flexible component, permitting, for example, proper packing of shell materials resulting in the formation of a high density shell structure, which can effectively contain the core binder and prevent its loss through diffusion and leaching process. The soft and flexible component in one embodiment is comprised of a polyether function. Specifically, in one embodiment there are disclosed in the aforementioned copending application encapsulated toners comprised of a core containing a polymer binder, pigment or dye particles, and thereover a shell preferably obtained by interfacial polymerization, which shell has incorporated therein a polyether structural moiety. Another specific embodiment of the copending application is directed to encapsulated toners comprised of a core of polymer binder, pigment, dye or mixtures thereof, and a polymeric shell of a polyether-incorporated polymer, such as a poly(ether urea), a poly(ether amide), a poly(ether ester), a poly(ether urethane), mixtures thereof, and the like.
Many of the prior art encapsulated toner compositions in particular colored toner compositions suffer from a number of deficiencies as indicated herein. For example, these toner compositions may not have the desirable fusing properties such as being able to be fused at reasonably low temperature of, for example, less than 160.degree. C.; they generally possess very low transparency projection efficiency either because of a significant difference in the refractive indices of the shell and core components or because of a poor colorant dispersion within the core; they usually require different or excessive amounts of charge control agents for different colored toners; and their rates of triboelectic charging are poor. In addition, some prior art colored encapsulated toners cannot be obtained in smaller toner size of, for example, less than 7 or 8 microns with a narrow size distribution of, for example, less than about 1.35 in a cost effective manner. Also, toner blocking or agglomeration may be a problem with several of the prior art encapsulated toners because of the porosity of the shell structure, especially when they are exposed to conditions of elevated temperatures. Further, some of the prior art colored encapsulated toners are comprised of colored pigment particles that may not completely be encapsulated by the shell, and the triboelectric charging effects of such pigments are, therefore, not fully passivated, and this would adversely affect and degrade the toner triboelectric characteristics, thereby causing image quality to deteriorate. In addition, many of the prior art toner compositions do not possess the necessary long-term physical and environmental stability. These and other disadvantages are eliminated or substantially eliminated with the process and toner compositions of the present invention. More specifically, thus with the toners of the present invention, the toner properties can in many instances be tailored to certain specifications. Specifically, with the toners of the present invention in embodiments, complete or substantial passivation of the triboelectric charging effects of the colorants is accomplished, and smaller toner particle size with narrow size distribution can be achieved without conventional classification techniques. In addition, excellent transparency projection efficiency can be obtained with the toners of the present invention in embodiments since the cellulose layer is very thin. Also, the toners of the present invention do not block or agglomerate over a extended period of time, for example up to six months, in embodiments. In addition, the processes of the present invention avoid the use of the relatively toxic polyisocyanates, polyacyl halides, polyamines, and the like, which are often utilized in the prior art processes for encapsulated toner compositions. Accordingly, the processes of the present invention in many embodiments do not require the costly post reaction waste treatments.